A compound-eye distance measuring apparatus, which causes an imaging device including a pair of imaging optical systems to capture a measurement object and obtains two images that are either left and right images or top and bottom images, calculates a distance to the measurement object using the triangulation principle. Such a compound-eye distance measuring apparatus is used for inter-vehicle distance measurement for automobile, an autofocus system for camera, and a three-dimensional shape measuring system.
FIG. 15 is a diagram showing triangulation performed by a compound-eye distance measuring apparatus. In FIG. 15, G1 is an imaging lens of the first imaging optical system, N1 is an image sensor plane of the same, G2 is an imaging lens of the second imaging optical system, and N2 is an image sensor plane of the same. Here, when a point P on a measurement object O is a measurement point and the point is on an optical axis of the first imaging optical system, the first imaging optical system forms an image of the point P on an image sensor plane on an optical axis a1 and the second imaging optical system forms an image of the point P on an image sensor plane located at a distance from an optical axis a2 by Δ in a baseline direction. Here, when a distance from each of the imaging lenses to the point P is Z, a baseline length that is inter-optical-axis distance between the two imaging optical systems G1 and G2 is D, a focal length of each imaging lens is f (for both G1 and G2), and a disparity is Δ an approximate equation as shown by the following Equation 1 holds.Δ≈D·f/Z  Equation 1
It is possible to extract the distance Z to the point P by modifying Equation 1, because Δ can be extracted by performing pattern matching an image obtained by the first imaging optical system and an image obtained by the second imaging optical system. A degree of correlation of the pattern matching can be determined with evaluation function SAD (Sum of Absolute Differences) that is a sum of differences (absolute value) in brightness of each of pixels between a small region in a standard image obtained by the first imaging optical system and a small region in a reference image obtained by the second imaging optical system. Here, when a calculation block size of the small region is m×n pixels, the SAD can be determined with the following Equation 2.
                    [                  Math          .                                          ⁢          1                ]                                                                      ∑                      i            =            0                                m            -            1                          ⁢                              ∑                          j              =              0                                      n              -              1                                ⁢                                                                I                ⁢                                                                  ⁢                0                ⁢                                  (                                                            x                      +                      i                                        ,                                          y                      +                      j                                                        )                                            -                              I                ⁢                                                                  ⁢                1                ⁢                                  (                                                            x                      +                      dx                      +                      i                                        ,                                          y                      +                      j                                                        )                                                                                                    Equation        ⁢                                  ⁢        2            
In Equation 2, x and y are coordinates on each of the image sensor planes, and I0 and I1 are a brightness value of the standard image and a brightness value of the reference image which are on coordinates in parenthesis, respectively. FIG. 16 is a diagram showing SAD calculation. The SAD calculation is performed while a position of a search block region in the reference image is displaced, with respect to a standard block region in the standard image, by dx in a baseline direction as shown in FIG. 16, and dx causing a SAD to be the minimum is the disparity Δ. It is possible to obtain distance information of all regions within a field of view, because a SAD can be calculated with any coordinates.
The compound-eye distance measuring apparatus needs testing whether or not distance measuring accuracy meets specific standards, because obtained distance information varies due to, for instance, variation in performance of an imaging optical system, an image sensor, or the like, assembly errors, and calibration errors.
Examples of a chart for evaluating the distance measuring accuracy of the compound-eye distance measuring apparatus include a chart C2 on which a two-tone lattice pattern is drawn as shown in FIG. 17, and a chart on which a multi-tone brightness pattern is randomly arranged and drawn as described in Patent Reference 1.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2001-091247